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Equivalent Circuit Diagram for a Capacitor

Thus far, we have considered capacitors and coils as "ideal" components possessing a pure reactance XC (capacitor) or XL (coil) in an AC circuit. In reality, however, both components also possess an active resistance which must be accounted for in detailed investigations. In such cases, instead of ideal components, an equivalent circuit diagram is substituted that includes the active resistance.

A capacitor charged by a DC voltage and subsequently disconnected from the voltage source loses its charge over a variable time period. Because the insulator between the capacitor plates is still conductive, albeit slightly, a correspondingly weak insulation current flows through it, by which the charges seek to equalise. This undesired leakage current in conjunction with the capacitor voltage results in a power loss termed insulation loss.

When a capacitor is connected to an AC voltage, the insulator's molecules are re-polarised continuously at the same frequency as the alternating voltage, causing a displacement current to flow in alternate directions. This results in a power loss dependent on the frequency and amplitude of the AC voltage as well as the capacitance of the component. This is termed dielectric loss.

Both losses can be accounted for by the following equivalent circuit diagram for a real capacitor. It consists of an ideal (loss-free) capacitance C connected in parallel with an ohmic resistance RV (representing the losses). This size of this resistance is such that the associated power loss is equal to the sum of the insulation and dielectric losses.